The present invention relates to the regeneration of elemental halogens from hydrogen halides and/or hydrohalic acids and more specifically to the use of a molten salt to catalytically oxidize hydrogen chloride to produce chlorine.
In the production of organic chlorides for use in plastics and other products, large amounts of hydrochloric acid are frequently produced as a by-product or waste. This excess hydrochloric acid is traditionally either utilized productively as is, if possible, or neutralized with limestone and discharged to the environment as an aqueous waste.
Due to increasing restrictions on the discharge of pollutants and the rising cost of chlorine it has become attractive to provide a method for regenerating chlorine from the by-product hydrochloric acid. Because the by-product hydrogen chloride from organic chlorinations contains only a few parts per million of sulfur impurities, it is potentially excellent feedstock for the efficient production of substantially sulfur-free elemental chlorine in a minimum number of steps. A number of previous attempts have been made to regenerate chlorine from hydrochloric acid. These methods have generally met with engineering and economic problems which have prevented their implementation on a large scale.
It is presently known that hydrogen chloride may be oxidized with sulfur trioxide to produce a mixture of chlorine and sulfur dioxide. This process requires a series of at least three separate process steps to obtain the mixture from which the chlorine must be subsequently separated.
A process taught by the West German patent No. 1,240,830 (Nonnenmacher), suffers from similar drawbacks. Hydrogen chloride is contacted with V.sub.2 O.sub.5, K.sub.2 S.sub.2 O.sub.7, and NaHSO.sub.4.H.sub.2 O which are supported on a finely divided silica gel. Due to this choice of salts and to the behavior of such catalysts when supported on finely divided carriers considerable amounts of the V.sub.2 O.sub.5 and sulfates decompose during the contacting. Concomitantly, substantial vanadium and sulfur oxide impurities are present in the product gas. To operate a process under these conditions, the catalyst material must be continuously replenished; and efforts are required to separate the added impurities in order to obtain a substantially pure chlorine product.
Another prior method is the oxidation of hydrochloric acid with sulfur dioxide and oxygen in the presence of a bed of metal oxide catalysts to produce sulfuric acid and chlorine. The solid supported catalysts used in such systems are difficult to prepare and tend to deactivate or degrade rapidly.
Oxygen has been used to oxidize hydrogen chloride in the presence of an oxide of nitrogen catalyst and an excess of sulfuric acid having a concentration of at least 65 percent. This process is complicated and expensive because it requires numerous process steps.
Hydrochloric acid has also been oxidized with oxygen in molten inorganic chlorides and an olefin chlorine acceptor such as ethylene. Such systems must accommodate complex chemical reactions which produce a variety of different reaction products. Also, because inorganic chlorides tend to volatilize in the reaction zone, the catalyst is driven off and separation of chlorine from the reactor effluent is different. Furthermore, operation with chloride salts is corrosive to process equipment.
In addition, hydrochloric acid has been regenerated by means of electrolytic processes which are expensive and which require substantial electrical energy consumption.